I've had a few instances were a channel would show black when I'd open up the "Live TV" window. Switching to another channel and back would clear it up.

I checked SiliconDust's site and their downloads page has a Firmware Changelog. The latest update listed this:

All models: Fix problem where sub-channels may not be reported or may fail to play in rare circumstances.

I'm fairly sure the channels I had the problem with were sub-channels, so I decided to see if my firmware was out of date. The download page has links for Windows, Mac and Linux. I clicked the Mac link and it downloaded their HD HomeRun Utilities package. I installed it and during installation process it saw that both my tuners had the 20120128 firmware so it updated them both to 20120405, the current version.

It hasn't happened since, but then again it didn't happen all that often to begin with so only time will tell if the problem has been resolved. SiliconDust HDHomeRun Dual black channel fix solution